Although a variety of chemical processes, such as oxidation, deamidation and aspartate isomerisation, may affect critical quality attributes of therapeutic proteins, such as antibodies, protein aggregation is arguably the most common process affecting protein stability. Aggregation is typically exacerbated and is the key degradation pathway of proteins formulated in aqueous solution at high concentrations, such as 10 mg/ml or greater. During storage, aggregation can lead to an unacceptably high level of high molecular weight species (HMWS) in the formulation or to formation of larger insoluble aggregates (particulates). Such contaminated formulations may fall outside the specification set by the U.S. Food and Drug Administration and other pharmaceutical regulatory authorities.
To some extent, protein aggregation can be controlled by optimization of various parameters of the protein composition. For example, methods to control the rate of aggregation may involve optimization of pH, addition of a metal ion chelator or addition of a surfactant.
The ionic strength of the composition can also affect the rate of aggregation in aqueous protein compositions. Conventional formulation development for a therapeutic protein therefore typically includes screening of tonicity modifiers, which can be selected from uncharged chemical species, such as sugars, or a charged chemical species, such as an inorganic or an organic salt. An uncharged tonicity modifier is typically preferred if the rate of aggregation is lower in low ionic strength compositions, while a charged tonicity modifier is preferred if the rate of aggregation is lower in higher ionic strength compositions. The charged tonicity modifiers typically used in aqueous protein compositions for therapeutic applications include sodium chloride. Typical uncharged tonicity modifiers include sucrose, trehalose, glycerol and mannitol.
Protein aggregation is a very complex process, involving a number of different mechanisms. However, it is believed that two dominant types of non-covalent interactions drive the protein aggregation: (1) hydrophobic interactions between non-polar parts of the protein molecules, and (2) charge-charge interactions between charged regions of the protein molecules. It is believed that in those cases where the rate of aggregation is lower in compositions of higher ionic strength than in compositions of lower ionic strength the key cause of aggregation is due to charge-charge interactions between the protein molecules.
However, it is also of critical importance that solutions and compositions which are capable of controlling protein aggregation exhibit a favourable toxicity profile, if they are to be of use in therapeutic applications. Thus, any additives which may be used to reduce the rate of protein aggregation must themselves have a favourable toxicity profile.
As such, there is a need for improved methods for preparing stable, highly concentrated protein solutions, particularly highly concentrated antibody solutions that have a favourable toxicity profile and are therefore suitable for use in therapeutic applications.
US 2007/0036866 (Kissel et al.) describes cationic block polymers comprising PEI and PEG residues.